Gold-catalyzed formation of pyrrolo- and indolo-oxazin-1-one derivatives: The key structure of some marine natural products

Article abstract of DOI:10.3762/bjoc.11.101

Various N-propargylpyrrole and indolecarboxylic acids were efficiently converted into 3,4-dihydropyrrolo- and indolo-oxazin-1-one derivatives by a gold(III)-catalyzed cyclization reaction. Some of the products underwent TFA-catalyzed double bond isomerization and some did not. Cyclization reactions in the presence of alcohol catalyzed by Au(I) resulted in the formation of hemiacetals after cascade reactions.

Full text of DOI:10.3762/bjoc.11.101

Gold-catalyzed formation of pyrrolo- and indolo-oxazin-1-one
derivatives: The key structure of some
marine natural products
Sultan Taskaya1, Nurettin Menges*1,2 and Metin Balci*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 897–905.
1Middle East Technical University, Department of Chemistry, Ankara,
doi:10.3762/bjoc.11.101
Turkey, 06800 and 2Yüzüncü Yil University, Faculty of Pharmacy,
Van, Turkey, 65100
Received: 10 February 2015
Accepted: 24 April 2015
Published: 28 May 2015
Email:
Nurettin Menges* - nurettinmenges@gmail.com;
Metin Balci* - mbalci@metu.edu.tr
Associate Editor: J. P. Wolfe
*
Corresponding author
© 2015 Taskaya et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
alkyne cyclization; gold-catalyzed reaction; indolo-oxazin-1-one;
marine natural products; pyrrolo-oxazin-1-one
Abstract
Various N-propargylpyrrole and indolecarboxylic acids were efficiently converted into 3,4-dihydropyrrolo- and indolo-oxazin-1-
one derivatives by a gold(III)-catalyzed cyclization reaction. Some of the products underwent TFA-catalyzed double bond isomeri-
zation and some did not. Cyclization reactions in the presence of alcohol catalyzed by Au(I) resulted in the formation of hemiace-
tals after cascade reactions.
Introduction
Pyrrole-containing heterocycles are widely distributed within a resistance reversal activity [12-14]. Lamellarins 4 with a
large number of natural products and biologically active mole- pyrrolo-oxazinone substructure are also marine natural prod-
cules [1]. These compounds possess a wide range of biological ucts and they inhibit the proliferation of cancer cells and there-
and pharmacological activities [2-5]. The oxazinone moiety is fore are promising candidates for anticancer drugs [15-20].
also frequently found in compounds displaying biological activ-
ities [6,7]. Pyrrolo-oxazinone structures with an oxazinone ring The design and synthesis of pharmaceutically relevant building
fused to a pyrrole are found in marine natural products, such as blocks has always been a significant goal in organic synthesis.
lukianol A (1) and lukianol B (2) (Figure 1) [8-10]. Lukianol B Moreover, modification of natural products is also an important
(
(
2) was found to be the most potent human aldose reductase approach to identify promising anticancer agents. Especially
h-ALR2) inhibitor in thousands of marine natural products due to the bioactivity of lukianols, many groups accomplished
screened [11]. The ningalin B alkaloid 3, having an isomeric the synthesis of these marine products and their analogues [21-
pyrrolo-oxazinone skeleton, possesses antitumor and multidrug 26]. Despite the general and widespread interest in these struc-
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Beilstein J. Org. Chem. 2015, 11, 897–905.
Figure 1: Structures of some marine natural products 1–4.
tures, the core structure, 1H-pyrrolo[2,1-c][1,4]oxazin-1-one Pyrrole and indole, bearing one (substituted) propargyl and
5), is not described in the literature. However, there are rela- carboxy group was initially designed as model substrate for the
(
tively few synthetic routes leading to the substituted core struc- construction of 6 and 7. Gold-catalyzed intramolecular cycliza-
ture 6 (Figure 2), which was generated by treatment of methyl tion of enyene carboxylic acids has been reported in the litera-
2
-pyrrole-carboxylate with chloroacetone [2,27-30]. This core ture to give the corresponding lactones [33-40]. For example,
structure is also a very important intermediate for the synthesis gold(I)-catalyzed reaction of 2-(phenylethynyl)benzoic acid (8)
of pyrrolo-pyrazinones as well as for pyrrolo-pyrazines [31]. yielded the exo- and endo-dig cyclization products, lactones 9
Recently, Wang et al. reported the base-catalyzed intramolec- and 10 in 62% yield in a ratio of 6:1 (Scheme 1) [41].
ular cyclization of alkynyl alcohols at high temperatures to give
1
,4-oxaza heterocycles [32].
Results and Discussion
The starting compound 11 was synthesized via a slightly modi-
fied route by acetylation of pyrrole with trichloroacetyl chlo-
ride in 99% yield [42,43]. The propargyl ester 13 [44-46] was
obtained in high yield from the reaction of 12 with propargyl
bromide in the presence of NaH as a base (Scheme 2). Gold
catalysis is an excellent method for constructing complex chem-
ical architectures in a mild manner that would be difficult to
achieve using other reactions [41,47-53]. Reaction of propargyl
ester 13 with AuCl3 resulted in the formation of 14, which is an
H2O (present in the reaction media) addition product to alkyne
Figure 2: Structures 5–7.
In this manuscript we envisaged a synthetic strategy leading to units. It has been proposed that a gold-activated water inter-
the core skeleton of marine products 6 and 7 (Figure 2) or their mediate can easily be added to alkynes. The addition of water to
analogues with short reaction times and an atom-economic alkyne has been reported by several research groups [54,55].
process using a gold-catalyzed alkyne cyclization reaction as The expected cyclization product having the structure 6 or 7
the key step.
was not formed.
Scheme 1: Intramolecular gold(I)-catalyzed cyclization reaction of 8 to give 9 and 10.
898

Beilstein J. Org. Chem. 2015, 11, 897–905.
Scheme 2: Synthesis of 13 and its reaction with AuCl3.
After failure of the cyclization reaction of 13 with AuCl3, even
in water-free solvents, we turned our attention to the construc-
tion of the pyrrolo-oxazin-1-one skeleton with the corres-
ponding acid 15. For this purpose, the propargyl ester 13 was
first hydrolyzed with K2CO3 to acid 15 (Scheme 3) [56]. The
acid 15 was reacted with various metal catalysts in chloroform
at different temperatures and for different reaction times. Five
different catalysts were tried (Table 1). Surprisingly, no reac-
tion was observed when the reaction was conducted with the
N-heterocyclic carbene (NHC) complex of Au(I) in chloroform
Table 1: Optimization of cyclization reaction of 15.
Entry Catalyst
Time
Temperature
(°C)
Yield
(%)
(h)
1
2
3
4
5
AuCl3
2
rt
rt
96
95
13
5
AgOTf
20
24
26
24
InCl3
50
50
rt
PtCl2(PPh)3
Au(L)a
No
reaction
(
Table 1, entry 5). Reactions with InCl3 and PtCl2(PPh3)3 as
catalysts gave very poor yields of exo-dig cyclization product 7
after 24 h (entries 3 and 4). AgOTf and AuCl3 were also
screened and AuCl3 was identified as the optimal choice due to
the shorter reaction time, high yield, and easy isolation of the
product 7 (entries 1 and 2, Table 1). We next investigated the
isomerization of the double bond in 7, which was treated with
trifluoroacetic acid (TFA) in chloroform at room temperature to
give 3-methyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (6) [27].
33 and N-propargylindole-2-carboxylic acids 37, 41 and 45,
which were synthesized by hydrolysis of the corresponding
After having obtained the optimal conditions for the esters (Table 2). The Sonogashira cross-coupling reaction [56-
Au-catalyzed cyclization of carboxylic acid 15, we attempted to 61] was used for the synthesis of the desired starting materials
determine the scope and limitation of this transformation. Then 29 and 33. For the Sonogashira coupling reaction we used a
we investigated the cyclization reaction of a range of substi- palladium catalyst and a copper(I) cocatalyst in the presence of
tuted N-propargyl-pyrrole-2-carboxylic acids 17, 21, 25, 29 and triphenylphosphine and triethylamine as the base. The
Scheme 3: Synthesis of 6.
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Beilstein J. Org. Chem. 2015, 11, 897–905.
carboxylic acids were then submitted to a gold-catalyzed corresponding oxazinones 18, 22, 26, 30, 34, 38, 42 and 46 in
cyclization reaction (Table 2).
high yields (Table 2). Substitution at the terminal carbon atom
of the propargyl group with methyl and phenyl groups did not
The optimized reaction conditions for cyclization was employed affect the mode of the cyclization reaction.
for pyrrole- and indole-carboxylic acids. We observed that all
substituted pyrrole as well as indole carboxylic acids under- Further investigation of cyclic products involved examining the
went a smooth gold-catalyzed cyclization reaction to afford the isomerization of the exocyclic double bond. For this purpose,
Table 2: AuCl3-catalyzed cyclization reaction of carboxylic acids.
Esters
Carboxylic acids
Cyclization products
Isomerization products
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Beilstein J. Org. Chem. 2015, 11, 897–905.
Table 2: AuCl3-catalyzed cyclization reaction of carboxylic acids. (continued)
Decomposition
the cyclization products were reacted with trifloroacetic acid Although 31 is thermodynamically more stable than 30, the
TFA) to form endo-cyclic double bonds. Some of the com- resistance of 30 to isomerization in the presence of TFA can be
(
pounds underwent double bond shifts to form an oxazinone explained by the regioselective protonation of the double bond.
skeleton 19, 23, 27, 39 and 43, whereas the compounds 30 and The proton is added to the carbon atom to generate the most
3
3
4 did not form the corresponding endo-cyclic systems 31 and stable carbocation. The most stable carbocation is the benzylic
5. In order to address this question, we carried out some DFT carbocation which would not undergo a rearrangement.
calculations [66].
As discussed above, when the cyclization reaction of 15 was
The calculations performed in the gase phase showed that 6 is carried out with [(NHC)AuCl] complexes (Table 1, entry 5), the
about 10.6 kcal/mol (DFT, B3LYP at 6-31G** level) and starting material was fully recovered. Gold N-heterocyclic
8
.1 kcal/mol (HF/6-311G** level) more stable than 7 carbene complexes, in conjunction with a silver salt, were found
Figure 3). However, in the case of 30 and 31, we also found to efficiently catalyze different types of reactions [67-69].
that the endo-cyclic isomer 31 is thermodynamically about Therefore, 15 was reacted with [(NHC)AuCl] complex in the
.8 kcal/mol (DFT, B3LYP at 6-31G** level) and 3.8 kcal/mol presence of AgOTf as the cocatalyst in chloroform. Beside the
(
4
(
HF/6-311G** level) more stable than the exo-cyclic isomer 30. expected cyclization product 7 (10%), the product 47 was
Figure 3: Geometry optimized structures of 6, 7, 30 and 31.
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Beilstein J. Org. Chem. 2015, 11, 897–905.
formed conclusively through incorporation of ethanol present in and 48 were formed in yields of 39% and 61%, respectively
chloroform (<1%) as the major product in 51% yield (Scheme 5).
(
Scheme 4). NMR spectral analysis of the mixture revealed also
the formation of 48 in 39% yield, which is the hydrolysis prod-
uct of 47. Attempts to isolate 47 failed. Chromatography on
silica gel converted 47 to 48 in almost quantitative yield.
However, the products 6 and 7 only were isolated when the
reaction was carried out in EtOH-free chloroform in 83% and
1
7% yields, respectively (Scheme 4).
Scheme 5: Reaction of 7 with Au(I)/AgOTf in the presence of EtOH.
To reveal the mechanism for the formation of the products we
carried out two different reactions. First, 15 was reacted with
CD3OD under the same reaction conditions. The 1H NMR spec- Based on all this information obtained, we propose the
tral analysis indicated the formation of 49 and 50 in 65% and following gold-catalyzed cascade reaction mechanism. The
3
5% yields, respectively. Furthermore, the amount of deuterium proposed catalytic cycle was initiated with π-activation of the
atoms in 49 and 50 attached to the methylene group as –CH2D triple bond by the carbene-based cationic gold species to form
was about 64% (Scheme 4).
the intermediate 51, which triggers a gold-promoted intramolec-
ular addition of a carboxy group to the alkyne functionality to
To prove whether 7 was involved as the intermediate, the give methylene-oxazin-1-one derivative 7 (or 18) (Scheme 6).
cyclization product 7 was reacted with Au(I)/AgOTf with EtOH In the next step, the double bond is activated by π-coordination
under the same reaction conditions. As a result, the products 47 of the gold(I) catalyst. This enables a nucleophilic attack by the
Scheme 4: Reaction of 15 with Au(I)/AgOTf in the presence of EtOH and CD3OD.
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Beilstein J. Org. Chem. 2015, 11, 897–905.
Scheme 6: Proposed reaction mechanism for the intramolecular gold-catalyzed cyclization followed by EtOH addition.
alcohol oxygen, which affords hemiacetal 47. In case of internal cyclization reactions in the presence of alcohol formed hemiac-
alkynes, two isomeric E- and Z-oxazinones during the cycliza- etal derivatives after gold-catalyzed cascade reactions.
tion reaction can be formed. However, we observed exclusive
formation of a single isomer. Recently, Michelet et al. [70] and
Supporting Information
others [41,71,72] demonstrated selectively the formation of
Z-lactones. The anti intramolecular addition of the carboxylic
Supporting Information File 1
acid to the gold-alkyne intermediate 51, forms an intermediate
2 where the substituent is in the endo-position. Removal of
Experimental and analytical data.
http://www.beilstein-journals.org/bjoc/content/
5
[
gold results in the formation of Z-isomer. Therefore, we assign
the endo-configuration to the formed oxazinones.
supplementary/1860-5397-11-101-S1.pdf]
Supporting Information File 2
NMR spectra.
Conclusion
[http://www.beilstein-journals.org/bjoc/content/
We have developed a general synthetic methodology of pyrrolo-
and indolo-oxazin-1-one derivatives. The key step was a gold-
catalyzed cyclization reaction of N-propargyl-substituted
pyrrole and indole carboxylic acid derivatives. The hydroxy
supplementary/1860-5397-11-101-S2.pdf]
groups of carboxylic acids attacked the activated triple bond to Acknowledgements
form 6-exo-dig cyclization products, oxazin-1-one derivatives. Financial support from the Scientific and Technological
Some of the exo-cyclic double bonds underwent isomerization Research Council of Turkey (TUBITAK, Grant No. TBAG-112
to endo-cyclic compounds upon treatment with TFA, while T360), the Turkish Academy of Sciences (TUBA), and Middle
some did not. DFT studies supported our findings. Moreover, East Technical University (METU) is gratefully acknowledged.
903

Beilstein J. Org. Chem. 2015, 11, 897–905.
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